FIG. 7 shows a known conventional diesel engine piston having a reentrant type combustion chamber, which is required to exhibit resistance to heat load, and a cooling structure for the reentrant type combustion chamber of the piston. Specifically, an annular cooling cavity 52 is formed in the body of the piston 50 around and outwardly of the outer periphery of a reentrant type combustion chamber 51, which is formed in the piston top face 55 and is eccentrically positioned with respect to the central longitudinal axis of the piston 50. A lower transverse wall 56 projects inwardly from the piston skirt toward the central longitudinal axis of the piston 50 and forms the bottom of the reentrant type combustion chamber 51. The top face 55 of the piston 50 projects radially inwardly beyond the maximum diameter of the reentrant type combustion chamber 51 so that the reentrant section 54, which is the junction of the top face 55 and the reentrant type combustion chamber 51, is an inwardly directed annular lip overhanging the outer portion of the reentrant type combustion chamber 51. During operation, a cooling liquid is supplied to the cooling cavity 52 through a cooling liquid inlet (not shown) so as to cool the piston top face 55, including especially the annular reentrant section 54 which becomes very hot. The cross-sectional area of the cooling cavity 52 in a plane containing the longitudinal axis of the piston 50 is larger near the bottom of the cooling cavity 52 than toward the top of the cooling cavity 52.
The conventional cooling structure of the diesel engine piston 50, however, presents the following problem. With an increasing output of the engine, the piston 50 tends to be subjected to a higher heat load. This causes the annular reentrant section 54 to become hotter and to incur deformation, cracking, melting, or the like. The result is a deteriorated durability of the reentrant type combustion chamber 51.